A slew rate limiter UGen. Limits the slope of an input signal. The slope is
expressed in units per second.
Since the UGen is initialized with the initial value of the input signal, some
tricks must be applied to set it to an alternative start value. For example:
val in = Select.kr(ToggleFF.kr(1), Seq("start".ir, "target".kr))
Slew.kr(in) // begins at "start" and moves towards "target"
maximum upward slope.
maximum downward slope.